The invention relates to imaging devices. More specifically, the invention relates to the compensation of nonuniform illumination provided by a bulb of an optical scanning device.
Commercial scanning products include flatbed scanners, scroll-fed scanners, hand-fed scanners, color copiers, fax machines and all-in-one multifunction products. The all-in-one products, which combine a scanning module with a printing module, can typically perform printing, scanning, faxing and copying. A typical commercial scanning product includes a bulb for applying illumination to an image to be captured, and imaging optics and a charge-coupled device (“CCD”) for capturing light reflected by the illuminated image.
Image quality depends upon the quality of the CCD and the quality of the imaging optics. The image quality also depends upon the quality of the bulb that illuminates the target area.
Perhaps the most variable and unpredictable element of the image capture is the bulb. Even if a constant input voltage is applied to the bulb, the brightness and color characteristics of the illumination can vary according to bulb temperature and other factors.
Xenon bulbs, hot cathode fluorescent bulbs and cold cathode fluorescent bulbs are typically considered for use in scanning products. Many scanner manufacturers prefer the cold cathode fluorescent bulbs over the Xenon bulbs and hot cathode fluorescent bulbs because the cold cathode fluorescent bulbs are less expensive and brighter.
However, scanner manufacturers are faced with considerations other than cost and brightness. Other important considerations include bulb longevity, power consumption and heat dissipation.
Bulb longevity can be increased, and power consumption and heat dissipation can be reduced, by turning off the bulb between scans or by supplying only a trickle current to the bulb between scans. However, if the bulb is turned off or if the trickle current is supplied to keep the bulb at a low level, the bulb is allowed to warm up after a new scan is commanded.
Xenon bulbs and hot cathode fluorescent bulbs typically warm up quickly; therefore, relatively little time would elapse if a scan commences at the end of the warm-up period. Cold cathode fluorescent bulbs, on the other hand, could take much longer to warm up. Waiting for a cold cathode fluorescent bulb to warm up could create an undesirably long waiting period. Therefore, the scanning usually commences during warm up, after a bulb shape has formed.
As scanning is being performed during bulb warm up, the intensity and spectrum of the illumination can drift. Therefore, compensation is performed to correct the drift in intensity and spectrum. The compensation is typically performed by adjusting bulb voltage and gain of the CCD's color channels.
The compensation is also performed after the bulb has warmed up, since drifts in intensity and color spectrum of the bulb can still occur. This is especially true for the cold cathode fluorescent bulbs.
The compensation should be accurate; otherwise, image quality and image capture accuracy will suffer. The compensation should also be relatively inexpensive to perform; otherwise, the cost of the scanning product will be increased. Manufacturers of high volume, low profit margin scanning products do not have the luxury of passing on cost increases to their customers. Even a seemingly trivial cost increase can significantly reduce the profitability of a product line in which millions of units are sold per year.